Heat exchanger with a liquid/gas mixing device with improved channel geometry

ABSTRACT

A heat exchanger with plates defining a first series of passages for channeling at least one refrigerant fluid and a second series of passages for channeling at least one calorigenic fluid, at least one passage of the first series being defined between a first plate defining an adjacent passage of the second series and a second plate. A mixing device is arranged in the passage of the first series and includes a first surface arranged facing the first plate and a second surface arranged facing the second plate, at least one first channel for channeling a gas phase of the refrigerant fluid, and at least one second channel for channeling a liquid phase of the refrigerant fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application No. PCT/FR2017/053505, filed Dec. 12, 2017, which claims priority to French Patent Application No. 1662581, filed Dec. 16, 2016, the entire contents of which are incorporated herein by reference.

Background

The present invention relates to a heat exchanger comprising series of passages for each of the fluids to be placed in a heat-exchange relationship, the exchanger comprising at least one mixing device configured to distribute at least one mixture having two liquid/gas phases into one of the series of passages.

In particular, the present invention may apply to a heat exchanger which vaporizes at least one flow of liquid-gas mixture, particularly a flow of multi-constituent mixture, for example a mixture of hydrocarbons, through exchange of heat with at least one other fluid, for example natural gas.

The technology commonly employed for an exchanger is that of aluminum brazed plate and fin exchangers, which make it possible to obtain devices that are highly compact and offer a large exchange surface area.

These exchangers comprise plates between which are inserted heat-exchange corrugations, formed of a succession of fins or corrugation legs, thus constituting a stack of vaporization passages and of condensation passages, one intended to vaporize refrigerant liquid and the other intended to condense a calorigenic gas. The exchanges of heat between the fluids may take place with or without phase change.

In order to ensure correct operation of an exchanger employing a liquid-gas mixture, the proportion of liquid phase and of gas phase needs to be the same in all of the passages and needs to be uniform within one same passage.

The dimensions of the exchanger are calculated on the assumption of a uniform distribution of the phases, and therefore of a single temperature at the end of vaporization of the liquid phase, equal to the dew point of the mixture.

In the case of a multi-constituent mixture, the temperature at the end of vaporization is going to depend on the proportion of liquid phase and of gas phase in the passages.

In the event of an unequal distribution of the two phases, the temperature profile of the refrigerant fluid is then going to vary according to the passages, or even vary within the one same passage. Because of this nonuniform distribution, there is the possibility that the calorigenic fluid(s) in a heat-exchange relationship with the two-phase mixture may have an exchanger outlet temperature that is higher than intended, and this consequently degrades the performance of the heat exchanger.

SUMMARY

One solution for distributing the liquid and gas phases of the mixture as uniformly as possible is to introduce them into the exchanger separately, then mix them together once they are inside the exchanger.

Document FR-A-2563620 describes such an exchanger in which a grooved bar is inserted into the series of passages which is intended to channel the two-phase mixture. This mixing device comprises separate inlets for a liquid phase and for a gas phase opening into a common mixing volume which is equipped with one outlet for distributing the liquid-gas mixture to the heat-exchange zone.

However, the liquid phase fed to the mixing device is then inevitably in a heat-exchange situation with the calorigenic fluid(s) circulating in the adjacent passages of the other series of passages. This may lead to a start of vaporization of the liquid phase actually within the corresponding inlets, thereby leading to an uneven distribution of the two phases of the mixture in certain passages of the series and in certain zones within one same passage.

In order to minimize the exchanges of heat that may occur at the mixing device, one solution might be to install the mixing device in a zone of the exchanger where no other fluid is circulating. The mixing device would then have to be positioned at one end of the exchanger, free of any means of discharging or of supplying fluid, and that would entail restructuring the exchanger in its entirety and would necessarily lead to an increase in the size thereof. Furthermore, such a solution does not allow the two-phase mixture to be introduced into the middle of the exchanger, something that may be desirable in instances in which the specifics of the method require this.

It is an object of the present invention to fully or partially solve the above-mentioned problems, notably by proposing a heat exchanger in which the distribution of the liquid and gas phases of a mixture is as uniform as possible, and to do so without excessively adding to the complexity of the structure of the exchanger, or increasing the size thereof.

The solution according to the invention is therefore a heat exchanger comprising several plates arranged parallel to one another so as to define a first series of passages for channeling at least one refrigerant fluid and a second series of passages for channeling at least one calorigenic fluid to be placed in a heat-exchange relationship with at least said refrigerant fluid, at least one passage of the first series being defined between a second plate defining an adjacent passage of the second series and a first plate, a mixing device also being arranged in said at least one passage of the first series and comprising:

at least one first channel for channeling a gas phase of the refrigerant fluid,

at least one second channel for channeling a liquid phase of the refrigerant fluid,

characterized in that the longitudinal section of the second channel, measured parallel to the second plate, decreases in the direction of said second plate.

Depending on the case, the exchanger of the invention may comprise one or more of the following technical features:

an orifice is arranged between the first channel and the second channel, said orifice comprising an inlet opening into the second channel and an outlet opening into the first channel, the longitudinal section of the second channel decreasing from the inlet of the orifice toward the second plate.

the first channel and the second channel extend parallel to the first and second plates.

the first channel is arranged between the second channel and the first plate,

the passages extend in a longitudinal direction, the first channel extending in the longitudinal direction and the second channel extending in a lateral direction orthogonal to the longitudinal direction.

the first channel is formed of a first cavity formed within the mixing device.

the mixing device comprises a first surface arranged facing the first plate and a second surface arranged facing the second plate, the first cavity opening onto the first surface.

the second channel is formed of a second cavity formed within the mixing device.

the second cavity opens onto the second surface.

the mixing device comprises several first longitudinal channels succeeding one another in the lateral direction.

the second channel comprises a first end situated at the level of the inlet of the orifice and a second end situated on the side of the second plate, the ratio between the longitudinal section of the second channel, measured at the second end, and the longitudinal section of the second channel, measured at the first end, being comprised between 0 and 0.8, preferably between 0.2 and 0.8.

the longitudinal section of the second channel (32) decreases progressively toward the second plate (2 b).

the second channel extends in the lateral direction, the cross section of the second channel being, in a plane perpendicular to the lateral direction, at least in part of frustoconical shape converging toward the second plate.

the reduction in the longitudinal section of the second channel is brought about by a lateral constriction of said second channel which occurs in the direction of the second plate.

the mixing device further comprises at least one third channel extending parallel to the first channel, said third channel being arranged between the second channel and the second plate,

The present invention may apply to a heat exchanger which vaporizes at least one flow of liquid-gas mixture, particularly a flow of multi-constituent mixture, for example a mixture of hydrocarbons, through exchange of heat with at least one other fluid, for example natural gas.

The expression “natural gas” relates to any composition containing hydrocarbons, including at least methane. This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be better understood by virtue of the following description, given solely by way of nonlimiting example and made with reference to the attached drawings among which:

FIG. 1 is a schematic view in cross section, on a plane parallel to the longitudinal and lateral directions, of part of a passage of the heat exchanger fed with a liquid gas two-phase mixture according to one embodiment of the invention;

FIG. 2 is a schematic view in cross section, on a plane parallel to the longitudinal direction and perpendicular to the lateral direction, of series of passages of the exchanger of FIG. 1;

FIG. 3A is a schematic view in cross section, on a plane perpendicular to that of FIG. 1, illustrating one embodiment of a mixing device with which an exchanger according to the invention is fitted;

FIG. 3B is a schematic view in cross section, on a plane perpendicular to that of FIG. 1, illustrating one embodiment of a mixing device with which an exchanger according to the invention is fitted;

FIG. 4A is a partial view of the mixing device of FIGS. 3A and 3B and of an alternative form of such a device;

FIG. 4B is a partial view of the mixing device of FIGS. 3A and 3B and of an alternative form of such a device;

FIG. 5 is a schematic view in cross section of mixing devices according to other embodiments of the invention.

FIG. 6 is a schematic view in cross section of mixing devices according to other embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate a heat exchanger 1 according to one embodiment of the invention, comprising a stack of plates 2 a, 2 b, 2 c . . . which extend in two dimensions: a longitudinal direction z, and a lateral direction y. The plates 2 a, 2 b, 2 c . . . are arranged parallel to and above one another, with a spacing, and thus form a plurality of passages for fluids in an indirect heat-exchange relationship via said plates. The lateral direction y is depicted as orthogonal to the longitudinal direction z and parallel to the plates 2 a, 2 b, 2 c . . . .

For preference, each passage has a flat and parallelepipedal shape. The separation between two successive plates is small in comparison with the length and the width of each successive plate.

The exchanger 1 may comprise a number of plates in excess of 20, or even in excess of 100, between them defining a first series of passages 10 for channeling at least one refrigerant fluid F1, and a second series of passages 20 (not visible in FIG. 1) for channeling at least one calorigenic fluid F2, the flow of said fluids being overall in the longitudinal direction z. The passages 10 of the first series may be arranged, all or some of them, to alternate with, or to be adjacent to, all or some of the passages 20 of the second series.

In a way known per se, the exchanger 1 comprises distribution and discharge means 43, 52 configured to distribute the various fluids selectively into the passages 10, 20 and to discharge said fluids from said passages 10, 20.

The sealing of the passages 10, 20 along the edges of the plates 2 a, is generally afforded by lateral and longitudinal sealing strips 4 attached to the plates 2 a, . . . . The lateral sealing strips 4 do not completely block the passages 10, 20 but advantageously leave fluid inlet and outlet openings in the diagonally opposite corners of the passages.

The openings of the passages 10 of the first series are arranged in coincidence one above the other, whereas the openings of the passages 20 of the second series are arranged in the opposite corners. The openings placed one above the other are respectively united with one another in manifolds 40, 45, 50, 55 of semi-tubular shape via which the fluids are distributed and discharged.

In the depictions of FIGS. 1 and 2, the semi-tubular manifolds 50, 45 are used to introduce the fluids into the exchanger 1, and the semi-tubular manifolds 40, 55 are used to discharge these fluids from the exchanger 1.

In this alternative form of embodiment, the manifold feeding one of the fluids and the manifold discharging the other fluid are situated at the one same end of the exchanger, the fluids F1, F2 thus flowing countercurrent-wise through the exchanger 1.

According to another alternative form of embodiment, the refrigerant and calorigenic fluids may equally circulate cocurrent-wise, the means feeding one of the fluids and the means discharging the other fluid then being situated at opposite ends of the exchanger 1.

For preference, the longitudinal direction is oriented vertically when the exchanger 1 is in operation. The refrigerant fluid F1 flows generally vertically and in the upward sense of that direction. Other directions and senses for the flow of the fluids F1, F2 are of course conceivable, without departing from the scope of the present invention.

It should be noted that, in the context of the invention, one or more refrigerant fluid(s) F1 and one or more calorigenic fluid(s) F2 of different natures may flow within the passages 10, 20 of the first and second series of the one same exchanger.

The distribution and discharge means 43, 52 advantageously comprise distribution corrugations 44, 51, 54, arranged between two successive plates 2 a, 2 b, . . . in the form of corrugated sheets, which extend from the inlet and outlet openings. The distribution corrugations 44, 51, 54 ensure the uniform distribution and recovery of the fluids across the entire width of the passages 10, 20.

Furthermore, the passages 10, 20 advantageously comprise heat-exchange structures arranged between the plates 2 a, 2 b, . . . . The purpose of these structures is to increase the heat-exchange surface area of the exchanger. Specifically, the heat-exchange structures are in contact with the fluids circulating in the passages and transfer thermal flux by conduction to the adjacent plates, to which they may be attached by brazing, thereby increasing the mechanical strength of the exchanger.

The heat-exchange structures also act as spacers between the plates, notably while the exchanger is being assembled by brazing, and in order to avoid any deformation of the plates during use of the pressurized fluids. They also provide guidance for the flows of fluid in the passages of the exchanger.

For preference, these structures comprise heat-exchange corrugations 11 which advantageously extend across the width and the length of the passages 10, 20, parallel to the plates, in the prolongation of the distribution corrugations 44, 51, 54 along the length of the passages 10, 20. The passages 10, 20 of the exchanger thus exhibit a main part of their length, constituting the heat-exchange part proper, which is covered with a heat-exchange structure, said main part being flanked by distribution parts covered with the distribution corrugations 44, 51, 54.

FIG. 1 illustrates a passage 10 of the first series 1, configured to distribute a refrigerant fluid F1 in the form of a two-phase liquid-gas mixture. The refrigerant fluid F1 is separated in a separator device 6 into a gas phase 61 and a liquid phase 62 which are introduced separately into the exchanger 1 via a lateral manifold 30 and the manifold 50. The two phases 61, 62 are then mixed with one another by means of a mixing device 3 arranged in the passage 10 and depicted schematically in FIG. 1. Advantageously, several passages 10, or even all of the passages 10 of the first series, comprise a mixing device 3.

FIG. 2 is a schematic view in cross section, on a plane parallel to the longitudinal direction z and perpendicular to the lateral direction y, of the exchanger of FIG. 1. It shows a stack of passages 10, 20 of the first and second series, mixing devices 3 being arranged in two passages 10.

The mixing device 3 according to the invention is advantageously made up of a bar, or rod, housed in a passage 10 and preferably extending in the section of the passage 10 over almost all, or even over all, of the height of the passage 10 such that the mixing device is in contact with each plate 2 a, 2 b that forms the passage 10.

The mixing device 3 is advantageously fixed to the adjacent plates 2 a and 2 b by brazing.

The mixing device 3 may exhibit, parallel to the longitudinal direction z, a first dimension comprised between 20 and 200 mm and, parallel to the lateral direction y, a second dimension comprised between 100 and 1400 mm.

As can be seen in FIGS. 3A and 3B, the mixing device 3 is notably delimited by a first surface 3 a arranged facing a first plate 2 a of the exchanger, and a second surface 3 b arranged facing a second plate 2 b. The second plate 2 b forms, with a third plate 2 c, the adjacent passage 20. The first and second surfaces 3 a, 3 b preferably extend roughly parallel, namely parallel or near-parallel, to the first and second plates 2 a and 2 b, respectively.

The mixing device 3 is advantageously of parallelepipedal overall shape. The first and second surfaces 3 a, 3 b are planar overall but may locally exhibit cavities forming fluid channels, as explained hereinafter.

The mixing device 3 comprises at least a first channel 31 for channeling a gas phase 61 of the refrigerant fluid F1, the direction of flow of the fluid being symbolized by the arrow 61.

Furthermore, at least one second channel 32 for channeling a liquid phase 62 of the refrigerant fluid F1.

According to the invention, the longitudinal section of the second channel 32 decreases in the direction of the second surface 3 b.

It should be noted that, in the context of the invention, the longitudinal section of the second channel 32, or of an opening of said channel, means the cross section of the channel measured parallel to the second surface 3 b, namely in planes of section of said channel that are parallel to the second plate 3 b.

Thus, in the embodiment illustrated in FIG. 3A, the first channel 31 extends in the longitudinal direction z, and the second channel 32 extends in the lateral direction y. The longitudinal section of the second channel 32 therefore decreases in the direction indicated by the arrow x.

By reducing the longitudinal section of the second channel 32 in the direction of the second plate 2 b, the area of contact between the liquid phase 62 and that part of the second plate 2 b that extends at the level of the mixing device 3 is reduced, thereby making it possible greatly to reduce the exchanges of heat that can occur between the calorigenic fluid F2 circulating in the adjacent passage 20 and the liquid phase 62 of the refrigerant fluid F1. This makes it possible to limit, or even avoid, vaporization of the liquid phase before it mixes with the gas phase of said refrigerant fluid F1. The two phases of the mixture are thus distributed as homogenously as possible actually inside the passages in the case of the two-phase mixture, and also between the various passages in the case of the two-phase mixture.

This solution offers the advantages of being simple to implement, of not altering the size of the exchanger, and of not making its structure more complex. Advantageously, the longitudinal channel 31 and the second channel 32 are in fluidic communication via at least one orifice 34 arranged between the first channel 31 and the second channel 32. The orifice 34 comprises an inlet 342 opening into the second channel 32, and an outlet 341 opening into the first channel 31. One or more orifices 34 may be arranged along the y-direction.

The longitudinal section of the second channel 32 decreases from the inlet 342 of the orifice 34 toward the second surface 3 b.

In operation, the mixing of the liquid phase 62 and gas phase 61 occurs overall downstream of the outlet 341 and the two-phase liquid/gas mixture is distributed from the mixing device via one or more passages 33.

The channels 31, 32 and/or the passages 33 may open onto the end faces 35, 36 of the mixing device 3, or onto faces that are set back from said faces 35, 36 toward the inside of the device 3.

For preference, the first and second channels 31, 32 are of slender shape, their length being large in comparison to their width.

Advantageously, the first and second channels 31, 32 cross the mixing device 3. Thus, the second channel 32 extends over almost all, or even over all, of the width of the passage 10, measured in the lateral direction y.

In the context of the invention, at least one passage 10 of the first series is defined between a first plate 2 a and a second plate 2 b, the first plate 2 a also defining an adjacent passage 20 of the second series immediately adjacent to the passage 10 concerned. A mixing device 3 is arranged in the passage 10 of the first series concerned.

Advantageously, the first channel 31 is formed of a cavity formed within the mixing device 3.

According to an alternative form illustrated in FIGS. 3A to 6, the first channel 31 may be formed of a cavity formed within the mixing device 3 and opening onto the first surface 3 a. For preference, the second channel 32 is formed of a cavity formed within the mixing device 3.

In one form of embodiment illustrated notably in FIG. 4A, the cavity that forms the second channel 32 opens onto the second surface 3 b. The second channel 32 then comprises an open second end 321 situated at the second surface 3 b.

According to an alternative form illustrated in FIG. 4B, the second channel 32 is formed by a blind internal cavity.

FIGS. 3A to 6 illustrate mixing devices 3 comprising a single second channel 32. The device 3 may also, advantageously, comprise several lateral channels 32 succeeding one another in the longitudinal direction z.

Likewise, the mixing device 3 may comprise one or more longitudinal channels 31. FIG. 3B illustrates a device 3 comprising a row of longitudinal channels 31 succeeding one another in the lateral direction y. For preference, the longitudinal channels 31 extend substantially parallel to one another. The first longitudinal channels 31 are advantageously arranged between the second channel 32 and the first surface 3 a.

More specifically, the second channel 32 advantageously comprises a first end 322 situated at the level of the inlet 342 of the orifice 34, and a second end 321 situated on the side of the second surface 3 b.

According to one advantageous embodiment of the invention, the longitudinal section of the second channel 32 decreases in such a way that the ratio between the longitudinal section of the second channel 32, measured at the second end 321, and the longitudinal section of the second channel 32, measured at the first end 322, is comprised between 0 and 0.8, preferably between 0.2 and 0.8.

Such sizing makes it possible to minimize the exchanges of heat between the liquid circulating in the second channel 32 and the adjacent fluids.

By way of example, in the configuration illustrated in FIGS. 4A or 4B, a ratio of longitudinal sections of the second channel 32 equal to 0 corresponds to a second channel 32 the cross section of which is triangular in shape.

In the case of a second channel 32 which is open-ended, the ratio between the longitudinal section of the opening 321 and the width of the second channel 32, measured at the first end 322, or bottom 322, is comprised between 0.2 and 0.8.

In particular, as illustrated in FIGS. 3A, 4A and 4B, the longitudinal section of the second channel 32 may decrease progressively toward the second surface 3 b.

According to one advantageous embodiment of the invention, and as visible in FIGS. 3A, 4A and 4B, the cross section of the second channel 32 is at least partially of frustoconical shape converging toward the second surface 3 b.

Alternatively, the reduction in longitudinal section of the second channel 32 may be brought about by a lateral constriction 324 of said second channel 32 in the direction of the second surface 3 b. What is meant by “constriction” is a sharp reduction in the width of the second channel 32, typically a reduction that is such that the ratio of longitudinal sections defined hereinabove is comprised between 0.2 and 0.8, this reduction taking place over a distance typically shorter than 4 mm, in the direction of the second surface 3 b.

In this way, the exchanges of heat that can take place between the calorigenic fluid F2 circulating in the adjacent passage 20 and the liquid phase of the refrigerant fluid F1 before it mixes with the gas phase are reduced still further.

For preference, the constriction 324 occurs substantially symmetrically.

Advantageously, the constriction is such that the second channel 32 has a cross section in the shape of an inverted T, as illustrated in FIGS. 5 and 6.

More specifically, the second channel 32 may comprise lateral walls 323 that are arranged perpendicular to the bottom 322 and said bottom 322 may be arranged parallel to the longitudinal direction z.

The representation of FIG. 3B remains applicable to a representation of the mixing device 3 in a plane perpendicular to that of FIG. 5 or 6.

According to one particular embodiment of the invention, illustrated in FIG. 6, the mixing device 3 further comprises a third channel 37 for channeling the gas phase 61 of the refrigerant fluid F1, said third channel 37 extending in the longitudinal direction z between the second channel 32 and the second surface 3 b.

The presence of this third channel 37 makes it possible to minimize still further the exchanges of heat between the liquid circulating in the second channel 32 and the fluids circulating in the adjacent passages. This in effect makes it possible to create a gas barrier which acts as a thermal insulator between the second channel and the second plate 2 b.

It being emphasized that the first channel 31 and the third channel 37 may have the same or different shapes and quantities. As shown in FIG. 6, the opening 321 of the second channel 32 advantageously opens into the third channel 37. In this embodiment, the mixing device 3 comprises at least two passages 33 for the two-phase liquid/gas mixture.

Of course, the invention is not restricted to the particular examples described and illustrated in the present application. Other alternative forms or embodiments within the competence of those skilled in the art may also be considered without departing from the scope of the invention.

For example, the exchanger according to the invention is chiefly described for the case in which the passages 10, 20 extend in the longitudinal direction z, the first channel 31 extending in the longitudinal direction z, and the second channel 32 extending in a lateral direction y orthogonal to the longitudinal direction z. The reverse is also conceivable, namely a first channel 31 extending in the lateral direction y, and a second channel 32 extending in the longitudinal direction z. The lateral direction y, and the longitudinal direction z, may also not be mutually orthogonal.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-15. (canceled)
 16. A heat exchanger comprising several plates arranged parallel to one another thereby defining a first series of passages for channeling at least one refrigerant fluid and a second series of passages for channeling at least one calorigenic fluid to be placed in a heat-exchange relationship with the at least one refrigerant fluid, at least one passage of the first series being defined between a second plate defining an adjacent passage of the second series and a first plate, a mixing device also being arranged in said at least one passage of the first series and comprising: at least one first channel for channeling a gas phase of the refrigerant fluid, at least one second channel for channeling a liquid phase of the refrigerant fluid, wherein a longitudinal section of the second channel, measured parallel to the second plate, decreases in the direction of said second plate.
 17. The exchanger as claimed in claim 16, wherein an orifice is arranged between the first channel and the second channel, said orifice comprising an inlet opening into the second channel and an outlet opening into the first channel, a longitudinal section of the second channel decreasing from the inlet of the orifice toward the second plate.
 18. The exchanger as claimed in claim 16, wherein the first channel and the second channel extend parallel to the first and second plates.
 19. The exchanger as claimed in claim 16, wherein the first channel is arranged between the second channel and the first plate.
 20. The exchanger as claimed in claim 16, wherein the passages extend in a longitudinal direction, the first channel extending in the longitudinal direction, and the second channel extending in a lateral direction orthogonal to the longitudinal direction.
 21. The exchanger as claimed in claim 16, wherein the first channel is formed of a first cavity formed within the mixing device.
 22. The exchanger as claimed in claim 21, wherein the mixing device comprises a first surface arranged facing the first plate and a second surface arranged facing the second plate, the first cavity opening onto the first surface.
 23. The exchanger as claimed in claim 16, wherein the second channel is formed of a second cavity formed within the mixing device.
 24. The exchanger as claimed in claim 23, wherein the second cavity formed within the mixing device opens onto the second surface.
 25. The exchanger as claimed in claim 16, wherein the mixing device comprises several first longitudinal channels succeeding one another in the lateral direction.
 26. The exchanger as claimed in claim 16, wherein the second channel comprises a first end situated at the level of the inlet of the orifice and a second end situated on the side of the second plate, the ratio between the longitudinal section of the second channel, measured at the second end, and the longitudinal section of the second channel, measured at the first end, being comprised between 0 and 0.8.
 27. The exchanger as claimed in claim 16, wherein the longitudinal section of the second channel decreases progressively toward the second plate.
 28. The exchanger as claimed in claim 16, wherein the second channel extends in the lateral direction, the cross section of the second channel being, in a plane perpendicular to the lateral direction, at least in part of frustoconical shape converging toward the second plate.
 29. The exchanger as claimed in claim 16, wherein the reduction in the longitudinal section of the second channel is brought about by a lateral constriction of said second channel which occurs in the direction of the second plate.
 30. The exchanger as claimed in claim 29, wherein the mixing device further comprises at least one third channel extending parallel to the first channel, said third channel being arranged between the second channel and the second plate. 